Download Scene Graph, Hierarchical Data Structures and Quaternions

Hierarchical Data Structures,
Scene Graph and Quaternion
Jian Huang, CS594, Fall 2002
This set of slides reference the text book and Comps
Graphics & Virtual Environments (Slater et. al, AddisonWesley), slides used at Princeton by Prof. Tom Funkhouser
and Gahli et al. SIGGRAPH course note #15 on OO API.
Spatial Data Structure
• Octree, Quadtree
• BSP tree
• K-D tree
Spatial Data Structures
• Data structures for efficiently storing geometric
information. They are useful for
– Collision detection (will the spaceships collide?)
– Location queries (which is the nearest post office?)
– Chemical simulations (which protein will this drug
molecule interact with?)
– Rendering (is this aircraft carrier on-screen?), and more
• Good data structures can give speed up rendering
by 10x, 100x, or more
Bounding Volume
• Simple notion: wrap things that are hard to check
for ray intersection in things that are easy to
check.
– Example: wrap a complicated polygonal mesh in a box.
Ray can’t hit the real object unless it hits the box
• Adds some overhead, but generally pays for itself .
• Can build bounding volume hierarchies
Bounding Volumes
• Choose Bounding Volume(s)
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Spheres
Boxes
Parallelepipeds
Oriented boxes
Ellipsoids
Convex hulls
Quad-trees
• Quad-tree is the 2-D
generalization of binary tree
– node (cell) is a square
– recursively split into four
equal sub-squares
– stop when leaves get “simple
enough”
Octrees
• Octree is the 3-D generalization of quad-tree
• node (cell) is a cube, recursively split into eight
equal sub- cubes
– stop splitting when the number of objects intersecting
the cell gets “small enough” or the tree depth exceeds a
limit
– internal nodes store pointers to children, leaves store
list of surfaces
• more expensive to traverse than a grid
• adapts to non-homogeneous, clumpy scenes better
K-D tree
• The K-D approach is to
make the problem space a
rectangular parallelepiped
whose sides are, in general,
of unequal length.
• The length of the sides is
the maximum spatial extent
of the particles in each
spatial dimension.
K-D tree
K-D Tree in 3-D
• Similarly, the problem
space in three
dimensions is a
parallelepiped whose
sides are the greatest
particle separation in
each of the three
spatial dimensions.
Motivation for Scene Graph
• Three-fold
– Performance
– Generality
– Ease of use
• How to model a scene ?
– Java3D, Open Inventor, Open Performer,
VRML, etc.
Scene Graph Example
Scene Graph Example
Scene Graph Example
Scene Graph Example
Scene Description
• Set of Primitives
• Specify for each primitive
• Transformation
• Lighting attributes
• Surface attributes
• Material (BRDF)
• Texture
• Texture transformation
Scene Graphs
• Scene Elements
– Interior Nodes
• Have children that inherit state
• transform, lights, fog, color, …
– Leaf nodes
• Terminal
• geometry, text
– Attributes
• Additional sharable state (textures)
Scene Element Class Hierarchy
Scene Graph
• Graph Representation
– What do edges mean?
– Inherit state along edges
• group all red object instances together
• group logical entities together
– parts of a car
– Capture intent with the structure
Scene Graph
• Inheritance -- Overloaded Term
– Behavior inheritance (subclassing)
• Benefit of OO design
– Implementation inheritance
• Perhaps provided by implementation language
• Not essential for a good API design
– Implied inheritance
• Designed into the API
Scene Graph
Scene Graph (VRML 2.0)
Example Scene Graph
Scene Graph Traversal
• Simulation
– Animation
• Intersection
– Collision detection
– Picking
• Image Generation
– Culling
– Detail elision
– Attributes
Scene Graph Considerations
• Functional Organization
– Semantics
• Bounding Volumes
– Culling
– Intersection
• Levels of Detail
– Detail elision
– Intersection
• Attribute Management
– Eliminate redundancies
Functional Organization
• Semantics:
– Logical parts
– Named parts
Functional Organization
• Articulated Transformations
– Animation
– Difficult to optimize animated objects
Bounding Volume Hierarchies
View Frustum Culling
Level Of Detail (LOD)
• Each LOD nodes
have distance
ranges
Attribute Management
• Minimize transformations
– Each transformation is expensive during rendering,
intersection, etc. Need automatic algorithms to
collapse/adjust transform hierarchy.
Attribute Management
• Minimize attribute changes
– Each state change is expensive during rendering
Question: How do you manage
your light sources?
• OpenGL supports only 8 lights. What if there are
200 lights? The modeler must ‘scope’ the lights in
the scene graph?
Sample Scene Graph
Think!
• How to handle optimization of scene graphs
with multiple competing goals
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–
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–
Function
Bounding volumes
Levels of Detail
Attributes
Scene Graphs Traversal
• Perform operations on graph with traversal
– Like STL iterator
– Visit all nodes
– Collect inherited state while traversing edges
• Also works on a sub-graph
Typical Traversal Operations
• Typical operations
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Render
Search (pick, find by name)
View-frustum cull
Tessellate
Preprocess (optimize)
Scene Graphs Organization
• Tree structure best
– No cycles for simple traversal
– Implied depth-first traversal (not essential)
– Includes lists, single node, etc as degenerate
trees
• If allow multiple references (instancing)
– Directed acyclic graph (DAG)
• Difficult to represent cell/portal structures
State Inheritance
• General (left to right, top to bottom, all state)
– Open Inventor
– Need Separator node to break inheritance
– Need to visit all children to determine final state
• Top to bottom only
– IRIS Performer, Java3D, …
– State can be determined by traversing path to node
Scene Graphs Appearance
Overrides
• One attempt to solve the “highlighting” problem
– After picking an object, want to display it differently
– Don’t want to explicitly edit and restore its appearance
– Use override node in the scene graph to override
appearance of children
• Only works if graph organization matches model
organization
Appearance Override
Multiple Referencing
(Instancing)
• Convenient for representing multiple
instances of an object
– rivet in a large assembly
• Save memory
• Need life-time management
– is the object still in use
– garbage collection, reference counts
Multiple Referencing
• Changes trees into DAGs
• Instance of an object represented by its path, (path
is like a mini-scene)
• Difficult to attach instance specific properties
– e.g., caching transform at leaf node
Other Scene Graph Organizations
• Logical structure (part, assembly, etc.)
– Used by modeling applications
• Topology structure, e.g., boundary
– surfaces, faces, edges, vertices
– Useful for CAD applications
• Behaviors, e.g., engine graph
• Environment graph (fog, lights, etc.)
• Scene graph is not just for rendering!!
Specifying Rotation
• How to parameterize rotation
– Traditional way: use Euler angles, rotation is specified
by using angles with respect to three mutually
perpendicular axes
• Roll, pitch and yaw angles (one matrix for each
Euler angle)
• Difficult for an animator to control all the angles
(practically unworkable)
– With a sequence of key frames, how to interpolate??
– Separating motion from path
• Better to use parameterized interpolation of
quaternions
Quaternion
• A way to specify rotation
• As an extension of complex numbers
• Quaternion:
u = (u0, u1, u2, u3) = u0 + iu1 + ju2 + ku3 = u0 + u
• Pure quaternion: u0 = 0
• Conjugate: u* = u0 - u
• Addition: u + v = (u0 +v0, u1+v1, u2+v2, u3+v3)
• Scalar multiplication: c.u = (cu0, cu1, cu2, cu3)
Quaternion multiplication
• uxv
= (u0 + iu1 + ju2 + ku3)x(v0 + iv1 + jv2 + kv3)
= [u0 v0 – (u.v)]+(uxv) + u0v + v0u
• The result is still a quaternion, this operation is not
commutative, but is associative
• u x u = - (u . u)
• u x u* = u02 + u12 + u22 + u32= |u|2
• Norm(u) = u/|u|
• Inverse quaternion:
u-1 = u*/|u|2, u x u-1 = u-1 x u = 1
Polar Representation of
Quaternion
• Unit quaternion: |u|2 = 1, normalize with
norm(u)
• For some theta, -pi < theta < pi, unit
quaternion, u:
|u|2 = cos2(theta) + sin2(theta)
u = u0 + |u|s, s = u/|u|
u = cos(theta) + ssin(theta)
Quaternion Rotation
• Suppose p is a vector (x,y,z), p is the
corresponding quaternion: p = 0 + p
• To rotate p about axis s (unit quaternion: u =
cos(theta) + ssin(theta)), by an angle of 2*theta, all
we need is : upu* (u x p x u*)
• A sequence of rotations:
– Just do: unun-1…u1pu*1…u*n-1u*n = 0 + p’
– Accordingly just concatenate all rotations together:
unun-1…u1
Quaternion Interpolation
• Quaternion and rotation matrix has a strict one-toone mapping (pp. 489, 3D Computer Graphics, Watt, 3rd
Ed)
• To achieve smooth interpolation of quaternion,
need spherical linear interpolation (slerp), (on pp.
489-490, 3D Computer Graphics, Watt, 3rd Ed)
– Unit quaternion form a hyper-sphere in 4D space
– Play with the hyper-angles in 4D
• Gotcha: you still have to figure out your up vector
correctly
More
• If you just need to consistently rotate an object on
the screen (like in your lab assignments), can do
without quaternion
– Only deal with a single rotation that essentially
corresponds to an orientation change
– Maps to a ‘hyper-line’ in a ‘transformed 4D space’
– Be careful about the UP vector
– Use the Arcball algorithm proposed by Ken Shoemaker
in 1985